The Top DSM Community on the Web

For 1990-1999 Mitsubishi Eclipse, Eagle Talon, Plymouth Laser, and Galant VR-4 Owners. Log in to remove most ads.

Please Support Morrison Fabrications
Please Support ExtremePSI

2G Piston to cylinder wall clearance?

This site may earn a commission from merchant
affiliate links, including eBay, Amazon, and others.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

mini zilo

15+ Year Contributor
783
16
Jul 17, 2006
Chicago, Illinois
I just picked up a 1997 7 bolt from a buddy of mine. It has never been started before, but it is a full short block. It has evo pistons and rods in it with new rings. The block has about 68,000 miles on it. Well, I pulled of the oil pan today to make sure everything was in spec. I checked end play, and it is in spec, I checked connecting rod to crank side clearance, and it is in spec. I also checked a few other things. The one thing that had me a little worried was the piston to wall clearance. Now these pistons are BRAND new and so are the rings, but the block has 68,000 on it and it was honed before this was put in. The spec for piston to wall clearance is something around .001-.002". With mine, I can fit a .004" feeler in between the piston and wall. What would happen if I ran the motor like this? Is this going to be a huge problem? I realize it is "trying to get away with something", but how serious is this? Thanks

Do you think the clearance might have something to do with the pistons from the Evo being made of a different metal, so it needs more clearance to expand or something like that?
 
Are the Evo pistons cast or forged? If cast, I think you may experience a little more blowby than normal, but I can't say at what clearance you'd start having issues. If forged, you're probably okay... but then again it also depends on the piston material. If they're an ultra low silica alloy, you could even be too tight and end up seizing it, it really all depends on the nature of the piston. The tell-all would be to find out what the Evo FSM recommended PTW is on the Evo those are from.

With a cast piston, I would expect a higher probability of some dipstick launching at moderate boost and some accelerated oil consumption at that 0.004". The rings will still be the biggest factor. Do you know the ring end gaps?
 
I have always viewed a 0.002" gap ok for stock compents. Forged stuff you can go up to 0.004". I have seen a forged piston engine with 0.002" gap and the pistons slapped like crazy.
 
Yes. Someone told me that is incorrect because the piston will move to the other side, but it still looks like I could fit a .004" feeler on each side of the piston.

That's the reason I asked. You're pretty much trying to measure a curved segment with a flat gauge, obviously it's not going to work.

What you really need to do is measure the bore of the cylinder wall and subtract the bore of the piston / 2. Do keep in mind that you'll have to make multiple measurements along the vertical plane of the bore.
 
Yes. Someone told me that is incorrect because the piston will move to the other side, but it still looks like I could fit a .004" feeler on each side of the piston.
The piston moving should not be a problem. The problem is if you're using a blade feeler gauge and not a wider and longer ribbon style feeler gauge. The blade gauge is too short and will resist curving to match the bore.

The old timers I've spoken to used ribbon feeler gauges to check PTW when they built engines. The way they did it was by turning the piston upside-down (so it's easy to hold onto), placing it in the bore and pulling it against the wall with one hand and drawing the ribbon gauge past it on the opposite side with the other hand. The weight of the pull resistance is measured with a scale, usually just a fishing scale and a steady hand. When the gauge that requires the correct amount of pull weight is found, you quit. Then of course, you'd divide the gauge thickness value by 2 for the radius difference.

What you really need to do is measure the bore of the cylinder wall and subtract the bore of the pistion / 2. Do keep in mind that you'll have to make multiple measurements along the vertical plane of the bore.
This is the best way to do it. A micrometer is much more precise than a feeler gauge for 99% of us.
 
Like blackspoolin said the only accurate way of checking the piston to wall clearance is with a dial bore gauge. For most engines .001"-.003" is fine on something with cast pistons. With the dial bore gauge you need to take 3 different measurements.

The first two will be about an inch down in the hole. Take your first meansurement and then spin the dial bore gauge 180 degreez and take another measurement. This will make your that your hole is round. Then your last measurement go about and inch up from the bottom of the hole and take a measurement. Now your first two number should be the same as the last meansurement. If your last measurement is different then it means that your cylinder is tappered.
 
Support Vendors who Support the DSM Community
Boosted Fabrication ECM Tuning ExtremePSI Fuel Injector Clinic Innovation Products Jacks Transmissions JNZ Tuning Kiggly Racing Morrison Fabrications MyMitsubishiStore.com RixRacing RockAuto RTM Racing STM Tuned

Latest posts

Build Thread Updates

Vendor Updates

Latest Classifieds

Back
Top